cw1200: fix missing unlock on error in cw1200_hw_scan()

Add the missing unlock before return from function cw1200_hw_scan()
in the error handling case.

Fixes: 4f68ef64 ("cw1200: Fix concurrency use-after-free bugs in cw1200_hw_scan()")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>